A magnetorheological (MR) fluid is a fluid with micron-sized magnetic particles that is configured to change viscosity almost instantaneously when subjected to a magnetic field. A typical MR fluid in the absence of a magnetic field has a readily measurable viscosity that is a function of its fluid components and particle composition, particle size, the particle loading, temperature and the like. However, in the presence of an applied magnetic field, the suspended particles appear to align or cluster and the fluid drastically thickens or gels. Its effective viscosity then is very high and a larger force, termed a yield stress, is required to promote flow in the fluid. The viscosity of the MR fluid increases to the point of becoming a viscoelastic solid by application of magnetic fields.
MR fluids exhibit an ability to change their rheology, and thus their flow characteristics, by several orders of magnitude in a timeframe on the order of milliseconds under the influence of an applied magnetic field. The induced rheological changes are completely reversible and, hence, can be utilized in devices that respond to the changes in the magnetic field environment. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable powertrain and engine mounts and in numerous electronically controlled force/torque transfer (clutch) devices. MR fluids offer significant advantages over other controllable fluids, such as ER fluids, particularly for automotive applications, because the MR fluids are generally less sensitive to common contaminants found in such environments, and they display greater differences in rheological properties in the presence of a modest applied field. The rheological properties of the carrier fluid, and the size and density of the suspended magnetic particles, define the important fluid characteristics, such as the settling rate, in devices that utilize MR fluids. Settling of the particles in an MR fluid significantly diminishes the performance of the fluid in the magnetized state, i.e., upon application of a magnetic field.
MR fluids are generally noncolloidal suspensions of finely divided (typically one to 100 micron diameter) low coercivity, magnetizable solids such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base carrier liquid such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a suitable magnetic field of, for example, about one Tesla. Since MR fluids contain noncolloidal solid particles which are often seven to eight times denser than the liquid phase in which they are suspended, suitable dispersions of the particles in the liquid phase must be prepared so that the particles do not settle appreciably upon standing nor do they irreversibly coagulate to form aggregates.
The magnetizable particles are kept in suspension, i.e., from settling, by dispersing a thixotropic agent, such as fumed or precipitated silica. Silicas stabilize the MR fluid by forming a network through hydrogen bonding between silica particles. This network breaks down under shear and reforms upon cessation of shear to keep the magnetizable particles suspended while exhibiting low viscosity under shear. Precipitated silica typically has a large particle size and low surface area due to its method of formation, whereas fumed silicas are typically smaller in size with larger surface area. Fumed silicas, when used, are typically surface treated. Both precipitated silicas and treated fumed silicas, however, often exhibit poor network formation, and consequently low yield stresses in the MR fluid during operation.
MR fluids may additionally contain surfactants to prevent coagulation and settling of the magnetizable particles. For example, the magnetizable particles may be coated with the surfactant. The surfactant is typically used in amounts less than 10% by weight relative to the weight of the silica. This typically translates to a concentration of less than 0.1% by weight of the fully formulated MR fluid. As the concentration of surfactant increases, the yield stress decreases. Yield stress is an indication of the strength of the silica network. While higher amounts of surfactant would be desirable, the amount of surfactant that may currently be used is limited due to its interference with the function of the thixotropic agent.
While the use of thixotropic agents and surfactants are effective to reduce the settling of the particles in MR fluids, they are not always sufficient to achieve the desired particle settling characteristics. Further, the use of these materials can also affect the magnetic response characteristics of the fluids, such as by reducing the magnetic saturation of the magnetizable particles.
Accordingly, it is desirable to provide MR fluids having suitable rheological and improved settling characteristics, while also maintaining the desired magnetic response characteristics of the magnetizable particles.